Warming to make US conditions more ripe for tornado-making storms, study says (+video)

Global warming weakens a key ingredient for tornado-making thunderstorms, prior studies held. True, researchers now say, but the number of days in the US where the right conditions exist will increase.

Conditions that spawn severe thunderstorms – including tornado-makers – across the US are expected to appear more frequently by the end of the century, according to new research.

Until now, studies of the potential impact of global warming on severe and tornadic thunderstorms generally have concluded that a key element needed – a sudden change in wind speed or direction with altitude known as vertical wind shear – would weaken on average as the climate warmed.

This would deprive thunderstorms of the opportunity to grow to the most intense levels, even as three other key ingredients – warm surface temperatures, lots of moisture in the air near the ground, and cold air at higher altitudes – were abundant. It was tough to say whether global warming held the potential to boost the number of severe thunderstorms.

The new research confirms the general pattern of weaker shear over the US, on average, in a warmer world. But when the scientists conducting the modeling study burrowed into the numbers behind the average, they found that as the climate warmed, shear tended to strengthen on days when the other ingredients also were abundant. These other ingredients determine the potential energy available to form a thunderstorm – energy that climate and weather researchers refer to as convective available potential energy, or CAPE.

Thus, while still in the minority in any given year, the number of high-CAPE days during the final years of the 21st century is projected to increase, compared with the conditions during the final 30 years of the 20th century.

"The net effect of warming is to increase the occurrence of days with high CAPE and sufficiently high shear to exceed the severe threshold" for thunderstorms, says Noah Diffenbaugh, a climate scientist and senior fellow at Stanford University's Woods Institute for the Environment, who led the team reporting the results Monday in the journal Proceedings of the National Academy of Sciences.

Weather forecasters define severe thunderstorms as storms that bring winds of at least 58 miles an hour, hail at least one inch across, or tornadoes.

The researchers caution that the study does not try to project the number of severe thunderstorms likely to occur.